1. Field of the Invention
The present invention pertains to a field of inorganic chemical industry, and particularly to a reduction-oxidation furnace for thermally refining phosphorus.
2. Description of the Related Art
Methods for refining phosphorus from rock phosphate mainly comprise a wet refining method (wet process) in which the rock phosphate reacts with an inorganic acid (generally with sulphuric acid) to produce rough phosphoric acid after phosphogypsum and impurities are separated, and a thermal refining method (thermal process), such as using an electric furnace or a kiln, in which gaseous elemental phosphorus is reduced by heating the rock phosphate and then is oxidized and hydrated to produce phosphoric acid.
The wet refining method has an advantage of low energy consumption. However, phosphoric acid produced by the wet refining method is not sufficient in purity and should be further purified. When a thermal refining method, such as using an electrical furnace, is employed, it is difficult to melt the rock phosphate, consumption of electric energy is large, and high grade rock phosphate is required, and low grade rock phosphate can not be processed. When a kiln, that is a hot method, is used, its reaction rate is low and a ring tends to be formed so that utilization ratio of the kiln is reduced and it is difficult to achieve a production at scale. Furthermore, in a conventional thermal refining method, gaseous phosphorus pentaoxide produced by oxidization is mixed in fumes having a complex composition so that it is difficult to separate the phosphorus pentaoxide from the fumes, purity of the phosphorus pentaoxide is decreased, and cost is high.
Chinese patent publication CN1160018A disclosed a process for producing phosphoric acid and phosphate by a thermal refining method. Injection metallurgy, melting reduction, and preparation of phosphoric acid by means of oxidative combustion of coal gas and elemental phosphorus are combined in the process. In other words, coal, oxygen and rock phosphate are injected. Reduction of the elemental phosphorus is performed in a melted state and oxidative combustion of the elemental phosphorus is performed in the same reactor so that phosphoric acid or phosphate is produced. However, the process has the following disadvantages.
Since the melting, reduction, and oxidization of the rock phosphate are simultaneously performed in the same reactor, fumes generated during the melting and fumes generated during the reduction and oxidization are mixed together so that impurities in the fumes are large. Therefore, it is difficult to extract phosphorus pentaoxide and purity of the phosphorus pentaoxide is low.
Since the rock phosphate is injected into a reduction-oxidation furnace body through a top blow gun, requirement for particle size of the rock phosphate is high, and cost is increased.
Since the top blow gun is immerged below a surface of liquid slag, it severely consumes itself due to hot corrosion and a top blowing pressure must increase. Moreover, water cooling is not suitable for the top blow gun immerged below the surface of the liquid slag. Otherwise, once water leaks, it is mixed into the slags. Explosion easily occurs to cause safety misadventure.
A bottom blow gun is disposed at a bottom of the reduction-oxidation furnace to inject coal powders and oxygen into melts. Therefore, when the bottom blow gun is replaced, operation of replacing the bottom blow gun can be performed only after stopping the furnace to empty the melts.
Since a flux inlet is disposed at a top of the reduction-oxidation furnace, it needs to be constantly opened and sealed in use. Its operation will be complex and it is difficult to seal the flux inlet.